No ipod touch game is long, but there are many that have replay value
I like pocket god becuz of the weekly updates
Mafia live/undead live will keep you entertained (don't even go near the knockoffs)
Bloons is fun for the price
And BIA is pretty long

rolando has great replay value because once you beat the game you can play for days trying to get all of the medals. and they release 2 more levels every week (until the summer when the sequel comes out)